A release of fine particulate from the Irving Oil refinery in east Saint John last weekend has been deemed "low risk" by the Department of Health.

The Department of Environment's investigation into the Nov. 12 incident that left some some homes and vehicles in the surrounding neighbourhoods covered in a white dust continues.

Irving Oil has not responded to a request for an interview.

But in an apology letter distributed to affected residents, the company said there was a "technical issue during the start-up of the refinery's fluidized catalytic cracking unit (FCCU) from the maintenance turnaround."

"As a result, we had an impact to some areas in the neighbouring communities with the distribution of fine particulate. We sincerely apologize."

The Department of Health investigated the release and subsequent air quality complaints and conducted a risk assessment.

"Based on information received from the Department of Environment, the Department of Health has determined that the release was a low risk to the health of those living in the communities affected," spokesperson Paul Bradley said in an email to CBC News on Friday.

Irving Oil was expected to submit a report to the Department of Environment by the end of the day Friday.

The report will be reviewed as part of the investigation, said department spokesperson Sheila Legacé.

The Department of Environment defines fine particulate as tiny airborne specks, measuring 2.5 microns in diameter or less, made up of solid or liquid matter, such as dust and soot. It is generated by natural sources, such as wind-blown dust and forest fires, and through fuel burning, especially fossil fuels and wood.

Fine particulate "causes and aggravates a variety of human breathing ailments, such as asthma, lung disease and bronchitis," according to the department. It also contributes to haze.

A New Brunswick Lung Association official has described fine particulate as a "serious threat to lung health."

Fine particulate "can be inhaled deeply into the lungs and damage lung tissue," the association's director of health Barbara Walls has said.

Clean air activist Gordon Dalzell, who used to sit on Irving Oil's community liaison committee, has also voiced concerns about fine particulate and the heavy metals it contains.